Friday, February 14, 2014

The "rules" allowing color reconstruction from the shape of melanin-containing organelles originate with feathered dinosaurs, and are associated with an increase in melanosome diversity. However, fuzzy dinosaurs like T. rex and Sinosauropteryx show a pattern found in other amniotes like lizards and crocodilians in which a limited diversity of shapes doesn't allow color reconstruction. An explosion in the distribution of the shapes of melanin-containing organelles preserved in living taxa and the fossil record may point to a key physiological shift within feathered dinosaurs.DOI: dx.doi.org/10.1038/nature12973

Inference of colour patterning in extinct dinosaurs has been based on the relationship between the morphology of melanin-containing organelles (melanosomes) and colour in extant bird feathers. When this relationship evolved relative to the origin of feathers and other novel integumentary structures, such as hair and filamentous body covering in extinct archosaurs, has not been evaluated. Here we sample melanosomes from the integument of 181 extant amniote taxa and 13 lizard, turtle, dinosaur and pterosaur fossils from the Upper-Jurassic and Lower-Cretaceous of China. We find that in the lineage leading to birds, the observed increase in the diversity of melanosome morphologies appears abruptly, near the origin of pinnate feathers in maniraptoran dinosaurs. Similarly, mammals show an increased diversity of melanosome form compared to all ectothermic amniotes. In these two clades, mammals and maniraptoran dinosaurs including birds, melanosome form and colour are linked and colour reconstruction may be possible. By contrast, melanosomes in lizard, turtle and crocodilian skin, as well as the archosaurian filamentous body coverings (dinosaur ‘protofeathers’ and pterosaur ‘pycnofibres’), show a limited diversity of form that is uncorrelated with colour in extant taxa. These patterns may be explained by convergent changes in the key melanocortin system of mammals and birds, which is known to affect pleiotropically both melanin-based colouration and energetic processes such as metabolic rate in vertebrates, and may therefore support a significant physiological shift in maniraptoran dinosaurs.

These are two of the fossil specimens sampled from the Cretaceous and Jurassic of China. Fuzz-covered dinosaur Beipiaosaurus shows the rounder melanosomes seen in living lizards and crocodilians while the bird shows the unique skinny melanosomes seen in living mammals, birds and many of the studied feathered dinosaurs to date. Changes in the diversity of these melanin-containing organelles may show a physiological shift occurred in feathered dinosaurs closer to the origin of flight.

Revision to rules for color in dinosaurs suggests connection between color and physiology

New research revising rules on deciphering color in dinosaurs may provide a tool for understanding the evolutionary emergence of flight and changes in dinosaur physiology. While surveying melanosome shape in fossil and extant specimens, a research team unexpectedly discovered that ancient maniraptoran dinosaurs, paravians, and living mammals and birds uniquely shared the evolutionary development of diverse melanosome shapes related to color. The similarity could relate to a key shift in dinosaurian physiology.